Melikaiella flora (Hymenoptera: Cynipidae)
Live Oak Petiole Gall Wasp
Approx 1 mile north of Mill Creek Road, along dirt road in North Cow Mountain area., Mendocino County, California
October 14, 2013

A cut-open acorn from Quercus agrifolia or Q. wislizenii. The larvae inside are gall wasp larvae. Note the individual egg-shaped chambers that each larva is in.

Acorn weevil larvae are also frequently found in acorns, but do not make the egg-shaped larval chambers (and the larvae look a little different as well).